Typically, printed circuit boards are prepared by stacking a plurality of prepregs in various arrangements, followed by pressing at high temperatures (e.g., greater than 170° C.). The prepregs consist of a partially cured curable resin coated onto fiber reinforcement, typically glass. A “partially cured” curable resin is known in the art as a “B-staged” resin. This partial curing or “B-staging,” raises the glass transition temperature (Tg) of the thermoset above ambient temperature (20° C. to 30° C.); whereby the Tg can be from 30° C. to 100° C. so that the prepregs can be rolled up without sticking. When the prepregs are stacked, pressed, and heated to achieve final cure the resin can flow to consolidate the layers before final cure (typically, final cure occurs when the Tg of the composition does not vary by more than 5° C. as measured by differential scanning calorimetry).
The ability to B-stage a resin is essential to the process of manufacturing printed circuit boards. A B-staged resin is a resin that has a portion of its curable moieties reacted for example, anywhere from 1 mol % to 95 mol %, of the resin's curable moieties; and wherein the “gel point” of the resin has not been reached. (The gel point is defined as the point at which a liquid formulation resin begins to exhibit elastic properties and increased viscosity). The “gel point” of a curable resin is the point along the cure profile at which an infinite network forms. Although further cure can occur, the resin will no longer flow. A B-staged resin can melt and flow during subsequent processing and further heating. During this latter process, called “C-staging” or “final cure”, the resulting thermoset is crosslinked beyond the ‘gel point’ and will no longer flow. At this C-stage, for example, typically more than 90 mol % of the curable moieties of the resin have reacted.
There continues to be a need for B-staging curable resins for prepreg production in a multistage process. Current B-staging processes include promoting polymerization reaction of part of the resin starting material and suspending the polymerization reaction at an appropriate B-stage. A notable drawback to this process is lack of reproducibility to consistently reach the same B-stage or the same degree of polymerization.
For example, in the aerospace and sporting goods industry, where prepregs are typically made with carbon fibers, a process is used which involves using a hot-melt technique in order to impregnate the fibers with resin. The thickness of the prepreg is finely controlled by using calender rollers. No solvent is used in this process as structural composites made using these prepregs needs to be below a certain void content such as typically, <1%. The prepreg produced in this manner undergoes little cure (e.g., <30% of the reactive moieties) during the prepregging process. The level of tack of this prepreg is controlled primarily by controlling the viscosity of the starting formulation at the prepregging temperature and the storage temperature. The main disadvantage of this type of prepreg is the need to transport the prepreg in refrigerated or cryogenic containers in order to prevent the tacky prepreg from undergoing further cure. Continued cure results in a loss of tackiness as well as causes issues during the subsequent laminate cure. In addition, once the prepreg needs to be used, the prepreg has to be warmed up back up to ambient temperature which adds additional work (and cost) to the process cycle. Most often these prepregs are stacked in specific stacking sequences depending on the design requirements for a specific use; and then the prepregs are cured in an autoclave under heat and pressure. It would therefore be advantageous to provide a resin composition that can be used to form a tacky prepreg that is storage stable at ambient temperature. Typically, a storage stable resin composition includes a B-stageable material that will not continue to significantly crosslink during storage so as to facilitate shipping at ambient temperature.
In an attempt to provide a formulation to satisfy the needs of the industry, dual-cure formulations have been described in the prior art which involve the use of two different types of polymerization, such as radical addition and condensation polymerization. The formulations need to be exposed to two different stimuli such as thermal energy and electromagnetic radiation including ultraviolet, electron beam or microwave radiation. The dual-curing formulations usually require a blend of a heat activated curing agent, and at least one polyolefinically unsaturated monomer such as a polyacrylate curable by a UV source. Other formulations include components that have the required functionalities of the dual-curing formulation incorporated in the same polymer backbone.
For example, Nair, et al., “Dual-Cure Propargyl Novolac-Epoxy Resins: Synthesis and Properties,” Polymers & Polymer Composites 2004, 12, 43-53, describes a dual cure thermoset by reactive blending a partially propargylated oligomeric phenolic novolac (PPN) resin with an epoxy resin. Curing of the resin occurs through a phenol-epoxy reaction at 135° C. together with a Claisen rearrangement and the addition polymerization of propargyl ether groups at 235° C.
WO 2008/019149 A1 describes a composition that includes a first and a second curable material which are cured at a first and a second curable temperature, respectively. The first curable material includes an alcohol and an anhydride that may cure by heating to a particular first temperature, at which temperature the second curable material may not cure. Also, in this case, the type of energy (thermal or electromagnetic) and the amount of energy applied may or may not differ. Typically the second temperature range for the second reaction requires significantly higher temperature ranges which may be detrimental to certain compositions making the application of such process impracticable.
US 2010/0222461 A1 describes a polymer composition that includes an epoxy resin system and a dual curing system including one or more curing agents containing one or more hydrazine-based curing agents and one or more amine-based curing agents containing one or more amine functional groups. The hydrazine-amine curing system enables the polymer composition to achieve elevated levels of gellation or degree of cure at lower temperatures than are achievable with amine functional curing agents alone.